1. Field of the Invention
This invention relates generally to telecommunication, and, more particularly, to a wireless telecommunication system.
2. Description of the Related Art
Conventional circuit-switched wireless telecommunication networks allow a mobile unit to establish a fixed connection, or link, to the network via a radio bearer. For example, in the Universal Mobile Telecommunication System (UMTS) protocol, a first mobile unit may form a fixed wireless telecommunication link with a second mobile unit via an air interface to a first base station, a first mobile switching center, a first radio network controller, a Public Land Mobile Network (PLMN), a second radio network controller, a second mobile switching center, and a second base station. Although the link is generally referred to as a wireless telecommunication link, persons of ordinary skill in the art should appreciate that the circuit-switched network may include wired and/or wireless connections between some elements of the network.
A single base station can establish a plurality of concurrent wireless telecommunication links with a plurality of mobile units using a plurality of channels. For example, in a Code Division Multiple Access (CDMA) protocol, each mobile unit may be associated with one of a plurality of orthogonal codes. The number of codes (sometimes referred to as the “spreading factor”), and thus the number of concurrent radio bearers that may be established by the base station, is typically limited by the data transmission rate. In general, the higher the data transmission rate, the lower the number of codes that may be supported by a base station. For example, a single base station may support up to 128 codes when implementing an Adaptive Multi-Rate (AMR) coding scheme that uses a coding rate of approximately 12 kbps. However, persons of ordinary skill in the art should appreciate that other factors, such as interference limits, soft handover overhead, and the like, may also influence the number of codes.
Voice and data may also be transmitted over a packet-switched network using a Voice over Internet Protocol (often referred to as VoIP). The Voice over Internet Protocol is increasingly common, at least in part because VoIP can handle voice and data communications homogeneously. Moreover, transmitting voice with VoIP may help reduce operational costs. In addition to conventional land-line telephones and cellular telephones, VoIP may be used to incorporate voice communications in devices such as personal data assistants, laptop computers, desktop computers, and the like.
To implement VoIP in a conventional wireless telecommunication network, the speech data frames are embedded in Internet Protocol (IP) data packets. The IP packet headers may increase the size of the data packets that are transmitted by the wireless telecommunication network. For example, the IP packet header for a speech data packet formed according to IP version 6 and using AMR 12.2 kbps coding would be about 300% of the size of a normal speech data frame. Consequently, the capacity of a packet-switched wireless telecommunication network would be reduced by a factor of about 3 relative to a circuit-switched wireless telecommunication network. In some instances, the IP packet headers may be compressed to reduce the overhead associated with transmitting speech data using VoIP. One compression method reduces the size of the IP packet header by removing predictable and/or static information from the header. For example, the size of the IP packet header may be reduced from about 60 bytes to about 2-3 bytes by reducing the information in the IP packet header to a sequence number, a context identifier, and a time stamp.
Not all IP packet headers may be compressed. For example, uncompressed or partially compressed IP packet headers may be sent when a mobile unit is being initialized or resynchronized, or when the network is recovering from an error. The uncompressed, or partially compressed, IP packet headers may include static and/or dynamic information that may be used to resynchronize the mobile unit or to recover from one or more transmission errors. FIG. 1 conceptually illustrates a conventional data stream 100 including a plurality of compressed data packets 105 and an uncompressed data packet 110. The vertical axis 120 indicates the size of the data packets 105, 110 and the horizontal axis 125 indicates the transmission time (or time slot) associated with the data packets 105, 110. The wireless telecommunication network is not generally able to predict when (or in what time slot) an uncompressed, or partially compressed, IP packet header will be required, so sufficient bandwidth should be reserved to transmit the uncompressed, or partially compressed, IP packet header when needed and/or requested.
One conventional technique for reserving bandwidth for the data packets that include the uncompressed, or partially compressed, IP packet header is to allocate a radio bearer that supports the data transmission rate required to transmit the uncompressed, or partially compressed, IP packet header without delay. This technique wastes a significant portion of the wireless telecommunication network resources because the spreading factor is then determined based upon the worst-case scenario in which the data transmission rate requirement for all data packets is determined based on the data rate requirement of the uncompressed, or partially compressed, IP packet headers. For example, as discussed above, the uncompressed IP packet header may be about 300% of the size of a normal speech data frame, so the required data transmission rate may be about triple the data transmission rate required to transmit compressed IP packet headers. Consequently, the spreading factor of a packet-switched wireless telecommunication network may be reduced by a factor of about three relative to the spreading factor of a circuit-switched wireless telecommunication network. Overall, this may degrade the wireless telecommunication network throughput by about a factor of three.
An alternative technique for reserving bandwidth for the data packets that include the uncompressed, or partially compressed, IP packet header is to allocate primary and secondary radio bearers having primary and secondary scrambling codes, respectively. In this technique, compressed data is transmitted using the primary radio bearer. The uncompressed or partially compressed IP packet headers, as well as the associated packet payloads, are transmitted using both the primary and secondary radio bearers.
Allocation of primary and secondary channels to carry uncompressed or partially compressed data packets has not yet been adopted in practice or as a wireless telecommunication standard, at least in part because of a number of recognized disadvantages. For example, allocating primary and secondary radio bearers may require important changes in the physical layer structure necessary to support transmission using a secondary scrambling code for the secondary radio bearer, such as specifying the distribution of the data over the primary and secondary codes and/or radio bearers. For another example, a loss of orthogonality between the primary and secondary scrambling codes may degrade the physical layer performance of the secondary scrambling code relative to transmitting the same data over two channels using a single scrambling code. For yet another example, the potential effects of interactions between the secondary scrambling code and the compression algorithm, which applies another scrambling code, have not yet been addressed.
Furthermore, adding a secondary radio bearer having a second scrambling code might not be sufficient to transmit the uncompressed, or partially compressed, IP packet header. As discussed above, the uncompressed IP packet header typically requires nearly 3 times the bandwidth of the compressed packets. Consequently, a third scrambling code might be required to transmit the uncompressed, or partially compressed, IP packet header, which makes the proposal more complicated and much more unlikely to be accepted as part of wireless telecommunication standards such as the 3rd Generation Partnership Project.
The present invention is directed to addressing the effects of one or more of the problems set forth above.